1. Field of the Invention
This invention relates to seismic data gathering devices. More particularly, it relates to testing the impedances of the geophone channels of such devices. This invention is applicable to devices for gathering seismic data on land, as well as to marine seismic data gathering devices. However, for clarity the description herein will be directed primarily to land seismic data gathering devices, with the understanding that the invention is not limited to land applications.
2. Description of the Prior Art
A land seismic data gathering device comprises a plurality of geophone channels and a multi-channel signal recording and computing system which typically is mounted on a truck, and may be referred to simply as a recording and computing system. Each geophone channel includes one or more geophones and a plurality of wires connecting the geophones to the recording and computing system. Typically each channel includes two wires, which may be referred to as the high and low wire, respectively. Each wire has two ends, which may be referred to for convenience as the near end and the far end, respectively. With the recording and computing system as a reference point, the near end of each wire is connected to the recording and computing system and the geophones are connected to the wires at their far ends or at points between the two ends. The geophones detect vibrations in the earth and generate electric signals representative of such vibrations. The signals are transmitted by the wires to the recording and computing system for recordation and processing.
Current practice in the field calls for using up to several hundred geophone channels; each channel is connected to from one to several hundred geophones. The geophones are manually disposed on the earth by technicians who move the geophone wires and plant each geophone in the earth at a desired location.
The accuracy and ease of interpretation of the seismic data gathered by the recording and computing system depends to a great extent on whether the impedance of each geophone channel remains reasonably constant over time. Under normal conditions, the impedance of each geophone channel varies primarily with the length of its wires and with the condition of the geophones and their connections to the wires. Ordinarily, the lengths of the wires will remain constant. However, the handling of the geophone wires and of the individual geophones may damage the insulation and the wires, as well as the parts within the geophones, and thus change the impedance of the channel. Accordingly, it is desirable frequently to test the impedance of each geophone channel for the purpose of locating defective wires and geophones. A defective channel produces no signal or a distorted signal and, when such signal is mixed with the signals of the non-defective channels, the overall signals become distorted. Such distortions can seriously impair the usefulness of the gathered seismic data.
Each geophone channel has a known nominal impedance which is a function primarily of the number of its geophones and of its length. Further, each channel has a predetermined maximum acceptable impedance and minimum acceptable impedance. These maximum and minimum acceptable impedances define a tolerance range of acceptable variation from the nominal impedance. It is desirable to determine whether the actual impedance falls within such tolerance range. If the actual impedance falls outside of the tolerance range, then the seismic crew is alerted to the possibility of a faulty geophone, such as one with a defective coil, a short between a coil and the geophone's metallic housing, or an open or shorted wire, or the like. Such faults should be located and corrected before the primary task of gathering seismic data is continued.
A commonly practiced method for making such impedance tests involves using complex switching means together with a resistance meter or ohmmeter. The geophone channels are tested seriatim by impressing a test signal on each channel individually and measuring the resistance across each channel with the resistance meter. The switching means are used to connect and disconnect each geophone channel to and from the recording and computing system and to and from the resistance meter according to the channel to be tested. See, for example, U.S. Pat. No. 2,917,706 (1959) to Thompson and FIG. 1 and the accompanying description below.
Other proposed geophone channel impedance testing systems employ a response test wherein switching means are used first for disconnecting the geophone channels from the recording and computing system and then for connecting the channels to a signal generator which simultaneously transmits a test signal to all the channels. Thereafter the geophone channels are reconnected to the recording and computing system which simultaneously records the response signals of all the geophone channels. See, for example, U.S. Pat. No. 3,858,169 (1974) to Bardeen. See also U.S. Pat. No. 3,717,810 (1973) to Spanbauer, which proposes driving the geophone channels with a constant RMS voltage or current and deriving the impedance from a measurement of the RMS voltage of the other of the voltage or current. Another proposed system involves impressing AC and DC currents of predetermined amplitudes on said geophone channels and detecting the excess of peak voltage produced by the AC current over the DC voltage generated by the DC current. See U.S. Pat. No. 4,052,694 (1974) to Fredriksson.
There is a need for a simple method and apparatus for determining without the use of the recording and computing system whether the impedances of the geophone channels fall within acceptable limits. Preferably such method and apparatus will permit the simultaneous testing of the geophone channels.